I think Peter Jackson has some experience building those.
In all seriousness, the most difficult part of such a structure is waterproofing. We used a company called Superior Walls for a while, and had great results with them. They're a bit different than the poured in place walls that are more common, but when you think about the logic behind them, it quickly starts to make more sense. For example, they set all the concrete walls on gravel, and so does the railroad. It's inexpensive, carries incredible loads, and drains extremely well. There are more pros behind it, but you can research it more if you like. They might be worth a call to see if they'll make a custom ceiling as well.
1978 13BT-40, 70 series mechanicals
1966 FJ45 lwb stock
2003 100 stock
1974V8J40, 1992FJ80, 1985FJ60, 1971FJ55, 1978FJ40, 1974FJ40, 1997 LX450 all long gone.